As a consequence of pi delocalization, aryl substituted azo compounds have vivid colors, especially reds, oranges, and yellows. Therefore, they are used as dyes, commonly known as azo dyes.
For the printing of textile, many different azo dyes have been developed. Cotton is often printed with reactive dyes. Due to the presence of a reactive group in these dyes, they are able to form a covalent bond with a hydroxyl group in the cellulose of the cotton. Due to this bond, the dye stays bound to the textile during washing.
Commonly azo dyes are prepared by first reacting an aromatic amine with a nitrite to form a diazonium salt. This diazonium salt can then be coupled with a coupling component to form the azo dye. The preparation of concentrated solutions of azo dyes is difficult because the required diazotization and coupling reactions give rise to very viscous phases which lead to serious stirring problems, or even make stirring completely impossible. In practice, the diazotization and coupling reactions are therefore often carried out in dilute aqueous solutions, and the dye content is concentrated only afterwards. This results in the need for large equipment and high amounts of water, which need to be removed afterwards. This is undesirable from an economic and environmental point of view.
A further disadvantage of currently used synthesis methods is that high amounts and many types of inorganic salts are used during the synthesis, which remain in the dyes as inorganic impurities. For application as inks for digital textile printing, the presence of these salts is undesirable. These salts can deposit on the print head, which results in reduced print quality. Although it is possible to largely remove the salts from the dyes, e.g. by reverse osmosis, this results in an extra process step which requires the use of expensive equipment and vast amounts of water.
Besides salts, organic impurities such as side-products from the synthesis steps and unreacted reagents are often present in dyes produced by current synthesis methods. These side products may need to be removed, which results in extra process steps and the need for additional materials and equipment. Even if the side products do not need to be removed, the formation of side products is undesirable as it diminishes the yield of the dye. This needlessly increases the costs of the used reagents to achieve a certain synthesis yield.
In order to achieve the desired shades of a specific color, such as black, it is well known to mix different azo compounds. The color of a mixture is determined by the specifically used compounds, as well as their relative amounts. Even slight deviations in relative amounts, for example changing the ratio of two dyes from 2.5:1 to 2.4:1, can lead to color differences which are readily noticeable by the human eye. Mixtures of dyes are commonly prepared by mixing individually prepared dyes in the desired relative amounts. Needless to mention, many process and purification steps are required to produce a desired dye mixture. The resulting dye mixtures often contain a high amount of impurities in the form of salts and organic impurities.
In order to make the synthesis more efficient, preparing dye mixtures from common building blocks without intermediate purification steps is advantageous. Such reaction processes which are performed consecutively in the same reaction vessel are also identified as one-pot processes. It has previously been described to prepare dye mixtures by a one-pot process using appropriate mixtures of diazo and coupling components. For example, EP 0600322 A2 discloses such a one-pot process. Because the reaction steps for the synthesis of dye mixtures are performed consecutively, meaning without intermediate purification steps, the amount of isolation and purification steps is reduced. This results in a simpler and more efficient process. The dyes do not have to be produced in separate steps and the step of mixing the dyes in the required proportions can be skipped. However, in such combined syntheses, the formation of viscous phases may play an even bigger role.
Thereto, U.S. Pat. No. 5,508,389 describes the addition of viscosity reducing agents to reactions in which dye mixtures are prepared from common building blocks without intermediate purification steps. The use of viscosity reducing agents lessens the problem of viscous phase formation. However, these viscosity reducing agents form an impurity which needs to be removed before the dye can be used in an ink-jet printer, and their use is therefore not desirable.
Besides viscosity, also the stability of the reaction mixtures used in preparing azo dyes can be an issue. In order to prevent demixing of the different phases, U.S. Pat. No. 7,300,504 discloses the use of a high shear mixer in the synthesis of inkjet ink compositions comprising a liquid vehicle, e.g. water, and a modified azo pigment. The modified azo pigment comprises the reaction product of at least one diazonium reagent and at least one azo coupler. The inkjet ink composition does not include a separate dispersant which needs to be removed before the inkjet ink composition can be used. U.S. Pat. No. 7,300,504 does not disclose the synthesis of azo dye mixtures, nor does it disclose any viscosity related effects.
As mentioned above, the usual procedure for the preparation of azo dyes is to diazotize an amine employed as diazo component in a first step and then, in a second step, to react the diazotized amine with the appropriate coupling component. The diazotization is normally carried out in a mineral acid solution by adding an excess of nitrite, e.g. sodium nitrite. When diazotization is complete, the excess nitrite is preferably removed before the azo coupling takes place. This removal may be effected by adding a small amount of non-diazotized amine or by adding urea or amidosulfonic acid (sulfamic acid). This has for example been disclosed in EP 0795586 A2. However, as disclosed in U.S. Pat. No. 4,845,638 sulfamic acid acts rapidly, but a disadvantage is that if an excess of sulfamic acid is added, it may result in secondary reactions. Use of sulfamic acid for the destruction of nitrite is therefore not self-evident.
A particularly desirable dye mixture is a mixture of Reactive Black 5 (RB 5) and Reactive Orange 78 (RO 78). Such a mixture provides for deep shades of black.

Reactive Black 5 (RB5)

Reactive Orange 78 (RO 78)
The above structures can also exist in their salt forms, in which the H-atoms in the OSO3H groups are replaced with an alkali metal, such as lithium, sodium or potassium.
Mixtures of RB 5 and RO 78 for dyeing textiles are known from e.g. CN 105273439 A, JP 2001 172523 A, and JPH 08127730 A. However, these documents all relate to mixtures of individually prepared dyes, and as such these mixtures suffer from all the above mentioned disadvantages. For example, due to the individual synthesis of RB5 and RO78, more synthesis steps are needed to prepare the mixtures, thus preparation of the mixtures is not efficient. Moreover, the resulting mixtures of individually prepared dyes suffer from high impurity levels, such as high salt concentrations and other organic and/or inorganic impurities.